INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 4, No 5, 2014
© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0
Research article
ISSN 0976 – 4402
Plant litter turnover, soil chemical and physical properties in a Ghanaian
gold- mined soil revegetated with Acacia species
Frimpong, K. A.1, Afrifa, E. K.A.2, Ampofo E. A.1, Kwakye P. K.1
1- Department of Soil Science, University of Cape Coast, Ghana
2- Department of Environmental Science, University of Cape Coast, Ghana
[email protected]
doi: 10.6088/ijes.2014040404538
ABSTRACT
The effects of stand-age of Acacia sp on plant litter turnover and decomposition, soil physical
properties and nutrient elements were studied in revegetated gold-mined soils. Litter in the 3and 6-year plantation soils were significantly lower (P < 0.05) than the forest soil, whilst 9and 12-year old revegetated soils had litter significantly higher (P < 0.05) than the forest soil.
Biomass decomposition constants measured in all treatments were statistically similar,
ranging from 1.03 in the forest soil to 0.94 in the revegetated treatments. Total N
concentrations were higher in the forest, 12- year and 9- year revegetated treatments and
varied from 0.20% to 0.50%. Organic carbon concentrations followed the increasing trend 3year < 6- year < 9-year < 12- year < forest. Exchangeable K concentrations measured in all
the soils were lower than 0.6 cmol kg,-1. Field moisture capacity (12.6 – 20.3 %) and
infiltration rates (25.2 – 31.1 cm h-1) in the 3-year, 6-year and 9-year stands were
significantly lower than in the forest and 12-year stand whilst soil bulk density values were
higher in the 3-year, 6-year and 9-year than in the forest and 12- year stands. The study
indicates that after 12 years of revegetation with Acacia sp, soil physical characteristics such
as bulk density, moisture holding capacity and infiltration rates improved to levels
comparable to those found under un-mined soils and that revegetation is a good strategy for
restoring the fertility of gold-mined soils.
Keywords: Plant litter turnover, decomposition constant, Acacia sps., revegetation, mining.
1. Introduction
Anthropogenic environmental stresses including surface mining activities can disturb primary
productivity, biodiversity and soil nutrient cycling patterns, resulting in land degradation. The
exploitation of mineral resources often leads to extensive soil degradation through the
destruction of vegetation and alteration of microbial communities, resulting in low soil
fertility and productivity (Jackson et al., 1990), Physical disturbances of the top soil during
stripping, stockpiling and reinstatement have adverse implications for soil fertility as these
processes can cause negative changes in soil moisture holding and infiltration capacities, bulk
density and substantial nitrogen (N), phosphorus (P) and potassium (K) losses. The Food and
Agriculture Organisation estimated that between 1990 and 2005, gold mining activities in
Ghana contributed significantly to land degradation and loss of cultivable land, resulting in a
loss of 26 % of the forest cover and 15-20 % of arable land at the Tarkwa, Ayanfuri, Dunkwa,
Esaase and Bogoso mining areas in Ghana (FAO 2006), Reclamation of mined soils is critical
to restoring the ecological integrity and the agricultural productivity of the soils because any
restoration process which improves soil physical properties and the availability of major
nutritive soil elements particularly, N, P, and K enhances the fertility and productivity of the
Received on March 2014 Published on April 2014
987
Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
Acacia species
soil. On a smaller scale, soil fertility of mined soils can be restored by the addition of crop
residues and animal manures, which supply nitrogen and organic carbon sources to stimulate
microbial activity and increase soil nutrient availability. In areas where large scale mining
activities have seriously degraded the soil, re-vegetating the mined soils with plant species
that are adaptable to the physically and biochemically altered soil conditions appears to be the
most sustainable option for restoring soil productivity. According to Sheoran et al. (2010), the
re-vegetation strategy is widely accepted for mined soils restoration as the revegetated soils
become protected against the effects of accelerated erosion, which lowers soil nutrient
concentrations through increased soil loss. Further, the rapid decomposition of plant litter
due to enhanced microbial activity in revegetated soils promotes soil nutrient cycling and
increases soil nutrient concentrations in accordance with the principles of sustainable soil
ecology management. Many re-vegetation efforts have employed N-fixing, heavy metal –
and acid-tolerant leguminous species, grasses, herbs, and trees for effective reclamation of
acidic and heavy metal-bearing soils. Once the re-vegetation process has begun and plant
species have been established, the reclaimed soils need to be frequently assessed to establish
their capacity to support `crop production in terms of major soil nutrient element availability.
This study therefore examines how the age of the Acacia species affect litter turnover and
decomposition, soil physical properties and nutrient element (organic C, inorganic N and
available P and K) concentrations in a revegetated gold mined Ghanaian soil.
2. Material and methods
2.1 Experimental site
The study was conducted at Ayanfuri in the Central Region of Ghana (5o N 0o W), It lies
within the semi-deciduous forest zone of Ghana with a mean annual rainfall of 1500mm
exhibiting a double maxima pattern (Dickson and Benneh, 1988), Revegetation of the mined
soils at the study area had been done in phases between 1993 and 2003 following surface
mining activities by Anglogold Ashanti Ghana Limited. The site covers a total area of 400 m2.
The stockpiled native topsoil was back-filled before Acacia species (Acacia auriculiformis)
were planted at an average density of 650 trees per hectare.
2.2 Experimental procedure
Grid sampling was done at each site covering an area of 10 m × 10 m (100 m2), The
differently aged Acacia plantations: 3, 6, 9 and 12, respectively (in years) served as the
blocks, with each study plot further subdivided into 4 sub-plots of 25 m2 each to serve as
replications. A primary forest bordering the area, which had developed on soil with the same
parent material, was used as the control treatment.
2.2.1 Soils
The soils studied were moderately well-drained, gritty sandy loams and clays developed in
situ over deeply weathered coarse-grained biotite granite found on summits and upper slopes.
They belong to the Nta and Ofin associations and classified as Rhodic Ferralsols (FAO,
1988), The chemical and physical properties of the soils studied are summarized in Tables 2
and 3 respectively.
Frimpong, K. A. et al
International Journal of Environmental Sciences Volume 4 No.5, 2014
988
Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
Acacia species
Composite soil samples were obtained from each of the 5 sites using the systematic soil
sampling procedure described by Jacobson (2002), The soil samples were sieved through a 2
m mesh and repacked into sampling bags for laboratory analysis. The fine earth (< 2 mm)
fractions were used for analyses of selected physical and chemical properties whereas
undisturbed samples (core samples) were used to determine soil bulk density and infiltration
capacity.
2.2.2 Assessment of aboveground litter content
Within each of the subplots, 3 labeled, 1 m x 1 m x 0.3 m wooden litter traps were randomly
placed 1 m above the ground. Litter collected in the traps were removed once a week and put
into correspondingly labeled nylon bags. The litter collected were separated into leaves, twigs
(wood pieces < 1 cm in diameter), flowers and fruits and recorded as monthly totals every 28
days. Each litter component was oven-dried separately at 60 OC to a constant weight. The net
primary production was estimated by multiplying the sum of the dry weights of all the
components of the litter fall by 3.3 (Bray and Gorham, 1994),
2.2.3 Assessment of decomposition rate of leaf litter
Fifty grams of leaves collected from the forest floor were air dried and put into nylon mesh
bags of 0.2 m x 0.2 m dimensions (Mwiinga et al., 1994), In each of the revegetated subplots,
40 nylon mesh bags were buried at random to a depth of 10 cm. The nylon mesh bags were
covered with leaf litter to simulate a fairly good forest floor condition for maximum influence
of meso- and macro-fauna (Lisanework and Michelson, 1994), Each site was clearly marked
for easy retrieval after covering with litter. For each site, the weights of litter in 4 bags
containing 50 g litter each dried to a constant weight at 60 oC were measured and averaged to
represent the mean initial dry weight. Four bags were retrieved from each location at the end
of every month for 12 months.
Litter decay constant (k) was calculated using a single exponential decay function adopted
from Singh and Gupta (1997) as follows:
----------------------(1)
Where, k is the decomposition constant.
To calculate k, the fomula was transformed into the equation:
ln
, such that
Where,
Wo = initial weight of litter
Frimpong, K. A. et al
International Journal of Environmental Sciences Volume 4 No.5, 2014
989
Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
Acacia species
Wt = weight of litter remaining after time t.
2.3 Laboratory Investigations
The core method (Blake and Hartge, 1986) was used to determine bulk density. Field
moisture capacity was measured using a method adopted from Kutilek and Nielson (1994)
and the infiltration rate determined by the double ring infiltrometer method (Bouwer, 1986),
Soil texture was determined using the method of Bouyoucos (1962), Soil pH was determined
in distilled water using a Glass electrode- Calomel electrode (McLean, 1982) MV Pracitronic
pH meter. Soil pH was measured at a soil: water ratio of 1: 2. Organic carbon was determined
using the wet combustion method of Walkley and Black (1934), Total N was determined
using the method of Bremner and Mulvaney (1982), Available phosphorus was determined
using the method of Bray and Kurtz (1945), Exchangeable bases were extracted with 1.0 M
NH4OAc (pH 7.0) and K in the extract determined by flame photometry (Chapman and Pratt,
1961), Thomas’ (1982)’s method was used for the determination of exchangeable acidity.
2.4 Statistical analysis
Data obtained from the laboratory analysis and field work were subjected to statistical
analysis to determine the mean values and the standard error of the means. Differences
between the means were determined by analysis of variance (ANOVA) using the MINITAB
16 statistical software. Correlation analyses were conducted to determine the relationships
between age of vegetation, above ground litter content, soil organic carbon and total nitrogen
concentrations.
3. Results
3.1 Above-ground litter biomass production and decomposition rate
The amount of above-ground litter measured in the 3- and 6-year Acacia plantation soils were
significantly lower (P < 0.05) than in the forest, but those measured in the 9- and 12- year
plantation soils were significantly higher (P < 0.05) than in the forest soil. Among the
revegetated sites, the quantity of above ground litter followed the trend 12-year > 9-year > 6year > 3-year. Above-ground litterfall was generally higher in the dry season than in the wet
season.
The experiments indicated that biomass decomposition constant, biomass C and N
concentrations were not affected significantly by season. Biomass decomposition constant
measured in the study varied from 1.03 in the forest soil to 0.94 in the revegetated treatments
(Table 1), Biomass decomposition rates measured in all the treatments in this study were
statistically similar. Organic C concentrations found in the forest litter were similar to those
measured in all the Acacia revegetated treatments. Total N concentrations were however
higher in the forest, 12- year and 9- year revegetated treatments than in the 6-year and 3-year
revegetated treatments.
Frimpong, K. A. et al
International Journal of Environmental Sciences Volume 4 No.5, 2014
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Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
Acacia species
Table1: Annual litter fall, biomass decomposition constant, organic C and total N
concentrations of litter from forests and Acacia species from revegetated mined soils
Annual litterfall
(t ha-1)
Treatment
Dry
Wet
season
season
7.02 ±
Forest
8.82 ±0.59
0.61
4.26 ±
3.12 ±
3Y
0.08
0.26
8.12 ±
6.11 ±
6Y
0.49
0.207
9.79 ±
8.08 ±
9Y
0.20
0.41
10.62 ±
9.11 ±
12Y
0.15
0.21
Biomass
decomposition
constant
1.03 ±0.5
0.94 ±0.7
Organic
C
(%)
48.5
±6.2
42.1
±6.3
0.99 ± 0.6
44.5±5.3
0.97 ± 0.7
47.0±3.7
0.96 ± 0.6
47.6±6.4
Total N
(%)
C:N ratio
1.9 ± 0.2
25.5 ± 2.3
1.75
±0.55
1.82
±0.47
2.0
±0.69
2.18
±0.8
24.02
±0.6
24.2 ± 0.9
23.5 ±0.7
21.8 ± 0.7
3.2 Soil chemical characteristics
Selected soil chemical properties measured in the forest and revegetated soils are presented in
Table 2.
Table 2: Selected chemical properties of forest and revegetated mined soils
Treatment Season
pH
Forest
Wet
5.1
Forest
Dry
5.0
3Y
Wet
3.6
3Y
Dry
3.6
6Y
Wet
4.0
6Y
Dry
4.1
9Y
Wet
4.3
9Y
Dry
4.2
OC
%
1.15 ±
0.09
1.17 ±0.03
0.66 ±
0.13
0.72 ±
0.17
0.70 ±
0.10
0.79 ±
0.11
0.79 ±
0.20
0.82 ±
Exch.
acidity
cmol kg-1
CEC
cmol kg-1
1.4 ± 0.01
25.1 ± 1.2
1.3 ± 0.05
28.3 ± 2.2
3.1 ± 0.01
6.6 ± 2.4
3.2 ± 0.01
8.2 ± 1.9
2.3 ± 0.01
10.1 ± 1.3
2.4 ± 0.01
12.1 ± 2.0
1.30 ± 0.01
12.2 ± 3.3
1.75 ± 0.12
11.5 ± 0.03
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Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
Acacia species
0.19
12Y
Wet
5.6
1.02 ±0.19
1.79 ± 0.20
21.4 ± 0.02
12Y
Dry
5.5
1.07± 0.19
1.85 ± 0.13
19.5 ± 0.02
Soil pH was less than 6 in all the soils (Table 2), The soil pH measured in the 12 year revegetated site was not significantly different from that found in the forest soil, but soil pH of
the 3-, 6- and 9-year revegetated soils were significantly lower (P < 0.05) than in the forest
and 12 -year soils.
Total N concentrations measured across the soils varied from 0.20 to 0.50 % (Fig 1), The
forest soil recorded the highest (P < 0.05) total N concentration while the 3 –year revegetated
soils recorded the least (P < 0.05), Total N concentration in the 12-year revegetated soil was
significantly higher (P < 0.05) than the other revegetated soil and those measured in the 6and 9- year revegetated soils were significantly higher (P < 0.05) than in the 3-year
revegetated soils.
Total N concentration ( %)
0.6
0.5
Wet season
0.4
Dry season
0.3
0.2
0.1
0
Forest
3-year
6-year
9-year
12-year
Forest / age of re-vegetated soil
Figure 1: Percentage total N concentration in forest and revegetated mined soils
Total N concentrations ranged from 0.20 to 0.49 % in the dry season and were highest (P <
0.05) in the forest soils while the least (P < 0.05) were found in the 3 -year revegetated soil.
However, no significant difference was found between total N concentrations in the forest
and 12-year revegetated soils.
During the wet season, NH4+ and NO3- concentrations measured in all the soils varied from
11.6 to 22.8 mgkg-1 soil and 6.2 to 14.1 mgkg-1 soil, respectively (Fig 2 a and b. ), In the dry
season NH4+ concentrations ranged from 27.4.2 to 11.7 mg kg-1 soil whiles NO3concentrations varied from 6.6 to 16.0 mg kg-1 soil. NH4+ and NO3- concentrations were
highest (P < 0.05) in the forest soil, but among the revegetated soils, NH4+ and NO3- generally
increased with age of the vegetation from 3 years to 12 years after planting
Frimpong, K. A. et al
International Journal of Environmental Sciences Volume 4 No.5, 2014
992
Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
Acacia species
30
Wet season
mg NH4+ - N kg-1 soil
25
Dry season
20
15
10
5
0
Forest
3-year
Forest
3-year
6-year
9-year
12-year
18
mg NO3 - - N kg-1 soil
16
14
12
10
8
6
4
2
0
6-year
9-year
12-year
Figure 2: NH4- -N and NO3- - N concentrations (mg kg-1) in forest and revegetated mined
soils
Organic carbon concentrations measured in soils from the various sites varied from 0.66 to
1.17 %, respectively (Table 2), Soil organic carbon concentration in the forest soil was
significantly higher (P < 0.05) than in all the Acacia plantation sites. Among the revegetated
soils, organic carbon concentrations followed the increasing trend 3- year < 6- year < 9-year
< 12- year revegetated soils, respectively. Furthermore, organic C concentration measured in
the forest and in all the revegetated soils were significantly higher (P < 0.05) in the dry
season than in the wet season.
The highest exchange acidity concentration (3.13 ± 0.01, P < 0.05) was measured in the 3
year revegetated treatment, followed by the 6 year revegetated treatment (2.28 ± 0.02) (Table
2), The least (P < 0.05) cation exchange capacity (CEC) was measured in the 3 year revegetated soils. CEC in the 6 year, 9 year and 12 year revegetated soils were similar, but
lower than that in the forest soil (Table 2),
Frimpong, K. A. et al
International Journal of Environmental Sciences Volume 4 No.5, 2014
993
Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
Acacia species
Available P concentrations measured during the wet and dry seasons in the forest soils, 6year, 9-year and 12-year soils were significantly higher (P < 0.05) than in the 3- year
revegetated soils.
Available P concentration (mg kg -1)
35
30
25
20
Wet season
15
Dry season
10
5
0
Forest
3-year
6-year
9-year
12-year
Forest / age of re-vegetated soil
Figure 3: Available P concentration (mg kg-1) in forest and revegetated mined soils
Exchangeable K concentrations measured in all the soils were lower than 0.6 cmol kg-1
(Fig.4), However, concentrations measured in the 12-year revegetated soils were similar to
those measured in the forest soils, and both were higher than in the other soils.
Exchanagebale concentration
( c mol kg -1)
0.7
0.6
0.5
0.4
Wet season
0.3
Dry season
0.2
0.1
0
Forest
3-year
6-year
9-year
12-year
Forest/ age of re-vegetated soil
Figure 4: Exchangeable K concentration (c mol kg-1 soil) in forest and revegetated mined
soils
Frimpong, K. A. et al
International Journal of Environmental Sciences Volume 4 No.5, 2014
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Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
Acacia species
3.3 Soil physical characteristics
Soil bulk density, field moisture capacity and infiltration rates determined in the forest and
revegetated soils are summarized in Table3.
Table 3: Selected physical properties of forest and revegetated mined soils
Treatmen
t
Sand
(%)
Silt
(%)
Clay
(%)
Bulk density
(Mgm-3)
Field
moisture
capacity (%)
Infiltration
rate
(cmh-1)
Forest
60
20
20
1.41
23.1
30.4
3Y
65
20
15
1.78
12.6
25.2
6Y
64
22
14
1.72
14.8
27.8
9Y
64
21
15
1.60
20.3
29.9
12Y
62
22
16
1.56
22.3
31.1
Soil bulk density, field moisture capacity and infiltration rates in the 12 year Acacia treatment
were all similar to values found in the forest soil. Field moisture capacity (12.6 – 20.3 %) and
infiltration rates (25.2 – 31.1 cm h-1) in the 3-year, 6-year and 9-year Acacia treatments were
significantly lower than in the forest and 12-year Acacia treatments whiles soil bulk density
values were higher in the 3-year, 6-year and 9-year Acacia treatments than in the forest and
12- year Acacia treatments, respectively.
4. Discussion
4.1 Aboveground litter fall
The study showed that aboveground litter fall increased with age of vegetation from 5.4 t ha-1
at the 3- year revegetated site to 10.3 t ha-1 at the 12 year revegetated site. This observation
supports an earlier report by Atwill and Leper (1987) that the amount of litterfall correlates
positively with tree size, which increases with age of the tree. Thus, older trees are expected
to produce higher quantities of litter compared to younger ones because canopies of trees
enlarge with age, resulting in increased production and dropping of litter. Acacia species have
been reported to be among the most suitable species that have been used for rehabilitating
mined soils (Jim, 2001) as they are fast-growing, able to produce high quantity and quality of
litter and tolerate a range of soil types and soil pH values (Yamamoto et al., 2003), The
quantity of aboveground litterfall measured in this study was up to 10 t ha-1 yr-1, which was in
accordance with the report by Bernhadt et al. (1993) that Acacia species can produce
between 7.5 to 9.0 t ha-1 of aboveground biomass annually. In this study the highest amount
of litterfall was measured in the 12 year revegetated treatment, but the amount of litterfall
measured in the 9 year revegetated treatment (8.29 t ha-1) compared favourably with findings
by Garity and Mercado (1994) and Saugier et al. (2001) who reported that most Acacia
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International Journal of Environmental Sciences Volume 4 No.5, 2014
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Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
Acacia species
species reach their optimum litter production between 7 and 9 years. In contrast, Kessler and
Breman (1996) found that Acacia species including A. nilotica, A. senegal and A. tortiliis
recorded their highest litter falls after 12 years of establishment. They pointed out that
differences exist in the growth patterns of various Acacia species and so their litter
contributions are likely to vary and further suggested that the differences in litter production
could also have been influenced by climatic and management conditions. According to
Holtgrieve et al. (1999) the annual quantity of litterfall is dependent on the proportion of
foliage biomass that dies within a year, which ultimately depends on such factors as rate of
leaf senescence, wind velocity, variation in canopy architecture and the tree species that make
up the forest (Jackson et al., 1990), In mature undisturbed forests the amount of litter that
accumulates on the forest floor depends upon the equilibrium between liter fall and the rate of
litter breakdown. According to Dutta (1999) litter fall acts as a critical regulating component
to enrich the microbial biomass of mined soils. In this study, microbial biomass carbon
positively correlated both with soil organic carbon (r2 = 0.84, P < 0.05) and litter organic
carbon content ( r2 = 0.97, P < 0.01), indicative that aboveground litter fall and subsequent
release of organic carbon following decomposition significantly influenced microbial activity
in the soil studied.
4.2 Decomposition of aboveground litter
In this study, the biomass decomposition constant measured in the forest soil was not
significantly different from those from the revegetated sites. It however appeared that across
the treatments, decomposition constants measured were higher in the wet season than in the
dry season (Table 1), Wang et al. (1999) defined decomposition of leaf litter as weight loss
due to a number of factors including the removal and/ or consumption of tissues by leaffeeding invertebrates, leaching, and biochemical degradation by microorganisms and
degradation during passage through the guts of invertebrates. Generally, warmer temperatures
and higher precipitation result in higher rates of decomposition, leading to less organic matter
accumulation. According to Vogt et al. (1995) these climatic factors interact with forest type,
substrate quality and nutrient availability in soils, obscuring patterns within similar climatic
and regional zones, but Zady et al. (1996) have noted that differences in decomposition rates
during various seasons were mainly due to variations in temperature. In this respect Agehara
and Warncke (2005) observed that litter decay under natural stands decrease by nearly 2% for
each 1o C drop in temperature, but both Graves (2000) and Smith (2004) pointed out that
precipitation was a more important factor governing litter decomposition than temperature.
and that the highest decomposition of evergreen forest litter occurred in the late rainy season,
concluding that litter disappearance in Nigeria was highest in the wet season due to activity
of mites and collembola.
4.3 Effect of aboveground litter fall on soil organic carbon (SOC) and total nitrogen
concentrations
Amounts of soil organic carbon vary considerably, ranging from 1 % or less in coarsetextured or highly eroded tropical soils to up to 3.5 % in prairie soils. However, SOC can be
Frimpong, K. A. et al
International Journal of Environmental Sciences Volume 4 No.5, 2014
996
Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
Acacia species
up to 10% in poorly drained soils (Hassink, 1996), SOC in terrestrial ecosystems are
determined by the difference between organic matter inputs and output (Gregorich et al.,
1998),
The influence of seasonal variations on SOC storage involves the relationship between annual
precipitation and temperature. Generally, wet and cool climates exhibit reduced
decomposition and promote organic matter accumulation, while warmer climates favour
accelerated decomposition of organic matter (Jenny et al., 1999), In contrast, Vanlauwe et al.
(1997) found no effects of climate on average SOC within given soil orders. Organic C levels
in mined soils are often low due to the disruption of ecosystem functioning, depletion of soil
organic pool (Duttal and Agrawal, 2002) and loss of litter layer during mining activities. Spur
et al. (2002) reported a strong relationship between litter decomposition and organic carbon
in both the soil of forest ecosystems and forests.
In this study, above ground litter fall correlated positively (R2 = 0.51 to 0.97, P < 0.05) with
organic carbon concentration in the soils across all the treatments (Table 4), According to
Bray and Graham (2004) litter fall constitutes a pathway for both energy and nutrient
transfer between plants and soils in forest land. Ovington (1994) also stated that much of the
carbon and energy fixed by forests is usually added to the forest floor through litter fall,
driving the C and N mineralization rates and influencing soil nutrient status. The
aboveground litterfall measured in the study also correlated positively (R2 = 0.57 to 0.66, P <
0.05) with total N concentrations measured in soils from all the treatments.
Table 4: Pearson correlations between amount of litter fall, liter properties and/ or selected
soil parameters and soil chemical properties
Relationships
Pearson Correlation
co-efficient (r)
P value
Litterfall and Total soil N
0.75
0.001
Litterfall and ECEC
0.97
0.000
Litterfall and Available Soil P
0.93
0.000
Litterfall and SOC
0.91
0.000
Litter O C and SOC
0.86
0.000
Total litter N and Soil Total N
0.53
0.041
Litter OC and MBC
SOC and MBC
0.97
0.84
0.000
0.000
MBC and Total Soil N
0.90
0.000
Soil pH and ECEC
0.93
0.000
Soil pH and Exch. Acidity
-0.74
0.002
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International Journal of Environmental Sciences Volume 4 No.5, 2014
997
Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
Acacia species
Litterfall positively correlated with total soil N (r = 0.75, P < 0.001), ECEC (r = 0.97, P <
0.001), available soil P (r = 0.93, P < 0.001) and SOC (r = 0.91, P < 0.001) Litter organic C
positively correlated with MBC (r = 0.84, P < 0.001), C: N ratio was less than 30:1 in all
treatments hence net N mineralization was expected.
4.4 Effect of revegetation on soil pH total N, inorganic N (NO3- and NH4+), available P
and exchangeable concentrations
The pH of the 3-year, 6-year and 9-year revegetated were less than 6 can be described as
strongly to very strongly acidic. This is reflective of the characteristic of a highly weathered
soil. When the pH of soils drop below 5.5, legume and forage growth is reduced due to metal
toxicities (such as aluminum or manganese toxicities), phosphorus fixation and reduced
population of N-fixing bacteria. This inhibits plant root growth. Maiti and Ghose (2005)
reported that pH vary from 4.9 to 5.3 in a mining dump site in India. In the 12-year and forest
sites, pH values were 6.0 and 6.1 respectively, indicative of the ability for the revegetation
strategy to improve soil pH. It must be emphasized however, that this effect is long term as
pH values were similar to the forest values only after a 12 year period of revegetation.
Despite the impressive litter production by Acacia species, in this study the contribution to
soil nitrogen was minimal to moderate, ranging from 0.35 to 0.41. Palm and Sanchez (1995)
posit that the Acacia species may compensate for this situation by their nitrogen fixing ability.
Previous authors including Coppin and Bradshaw (1982) and Sheoran et al. (2008) have
shown that major nutrients such as N, P and K concentrations in mined soils are usually low,
especially in the short term. In the long term, revegetation may result in increased soil
organic matter content through litter deposition and decomposition. Organic matter is the
major source of nitrogen, available P and K in unfertilized soils (Donahue et al., 1990),
Ghosh et. al. (1983) have suggested that organic carbon greater than 0.75% indicates good
fertility while Maiti and Ghose (2005) found that organic carbon is positively correlated with
available N and K and negatively correlated with Fe, Mn, Cu, and Zn. The increase in organic
carbon level is due to the accumulation of leaf litter and its decomposition to form humus.
Nutrient recycling and availability in reclaimed sites are reflected in part by the rate of
decomposition of plant material. Although oxidation of soil nitrogen to nitrate may be
impeded by acidic soils, C: N ratios of aboveground litter collected from each of the sites
were lower than 30, indicative of the potential for net N mineralization. Available N (NH4+
and NO3-) concentrations in all the revegetated soils were lower than in the forest soil. In this
study, bare mined soils were absent therefore, it would be difficult to ascertain whether N
mineralization or net N immobilization occurred in the revegetated soils following
decomposition of the aboveground litter. However, the NH4+ - and NO3- - N concentrations in
all the revegetated soils were low suggesting that notwithstanding the high rate of litter
turnover, mineral N availability was likely to be low.
Available P concentrations in all the soils studied were low, ranging from 9.3 – 19.0 mg kg-1.
The low available P concentrations may be attributable to the inherently low P levels in this
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highly weathered tropical soil, which might have been exacerbated by low soil pH. However,
the available P concentrations in the revegetated soils increased with increasing age of the
vegetation in the same way as soil pH increased.
The management implications of these observations are that the type of fertilizer and
application rates will vary, depending on the ages of revegetation activities. Cation exchange
capacity (CEC) of the 3-year, 6-year and 9-year revegetated soils were lower than 24 cmol
kg-1 soil, which is not uncommon in highly weathered, low pH tropical soils. CEC of the 12year revegetated and forest soils were 35 cmol kg-1 soil. The relatively higher CEC in these
soils is attributable to the high organic C contents in these soils. In the study, organic C
concentration positively correlated with CEC (r = 0.86, P < 0.001) and negatively with
exchange acidity (r2 = -0.74, P < 0.001), The low pH in the 3-year, 6-year and 9-year
revegetated soils could also have contributed to the low CEC in those soils as tropical soils
are dominated largely by pH-dependent or variable charges, which influence their charge
characteristics significantly (Frimpong et al., 2006), Maiti and Ghose (2005) concluded that it
is important to increase the pH and organic matter content of soils for sustainable reclamation
of mining overburdens.
4.5 Effect of re-vegetation on soil bulk density, infiltration rate and field moisture
capacity
Changes in soil physical properties such as increased aggregation and bulk density, and
reduced water infiltration and moisture-holding capacity associated with mined soil lower the
potential of such soils for crop production. The bulk densities in the 3-year, 6-year and 9-year
revegetated soils were up to 1.78 mgm-3, indicative that the soils may pose problems for root
establishment. The relatively high bulk density values in these soils are consistent with the
compact nature of mined soils due the presence of abundant rock fragments and concretions
in such soils (Sheoran et al., 2010), Bulk density of productive natural soils generally ranges
from 1.1 to 1.5 gcm-3. High bulk density limits rooting depth in mine soils. Plant roots are
reported to grow well in soils with bulk densities of up to 1.4 g cm-3 and that root penetration
is impeded significantly at bulk densities above 1.7 g cm-3. Soil compaction directly limits
plant growth, as most species are unable to extend roots effectively through high bulk-density
mine soils. The bulk densities in the forest and 12-year revegetated soils were lower than 1.55
gcm-3. This confirms that revegetation can improve soil stabilization soil through extensive
root systems development, increased soil organic matter, lower bulk density and moderate
soil pH, thereby improving soil nutrient availability. In revegetated soils, the plants
accumulate soil nutrients and redeposit them on the soil surface in the form of organic matter,
increasing soil nutrient availability following microbial breakdown (Li, 2006), According to
Dorbgetor et al. (2012), topsoil depth, bulk density, porosity, aggregate stability, water
content, soil strength, crushing and compaction of soils and water infiltration are all soil
properties that can be altered by management practices.
Field moisture capacity of the revegetated soils varied from 10.2% in the 3-year revegetated
sitel to 22.3 % in the 12-year revegetated site. The moisture holding capacity in the latter was
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Plant litter turnover, soil chemical and physical properties in a Ghanaian gold- mined soil revegetated with
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similar to that measured in the forest soil (23.1 %), Similarly, infiltration capacity also
increased with increasing age of vegetation at the various sites from 10.2 cm h-1 in the 3-year
revegetated soil to 15.1 cm h-1 in the 12-year revegetated soil. Infiltration rate in the forest
soil was 15.4 cm h-1. Increasing infiltration rates and moisture holding capacity could be as a
result of decreasing bulk densities in the revegetated soils due to increasing organic matter
from litter decomposition. Soil organic matter is responsible for binding soil particles
(Gregorich et al., 1989) and improvement of soil porosity, which controls the amount of
available water for vegetation (Davies and Younger, 1994), Soil microbial activity also
declines when soil layers are compacted (Edgerton et al., 1995), but soils with active
microbial communities are often well aggregated and vice versa. Soil microbes involved in
organic matter decomposition produce polysaccharides that improve soil aggregation and
plant growth (Williamson and Johnson, 1991),
Plant litter and root exudates provide nutrient-cycling to soil (Coates, 2005; Mertens et al.,
2007) and leguminous trees potentially improve soil fertility through numerous processes,
including maintenance or increase of soil organic matter, biological nitrogen fixation and
uptake of nutrients from soil horizons below the reach of roots of under-storey herbaceous
vegetation. They can also increase water infiltration and storage, reduce loss of nutrients by
erosion and leaching, improve soil physical properties, reduce soil acidity and improve soil
biological activity.
5. Conclusion
The study demonstrated that revegetation was a good strategy for restoring the fertility of
gold-mined soils, but that the effect of revegetation on aboveground litter biomass, N, P and
K concentration varied with the age of the revegetated soil. The study further showed that
after 12 years of revegetation with Acacia species the soil physical characteristics such as
bulk density, moisture holding capacity and infiltration rates improved to levels comparable
to those found under un-mined soils. These observations were attributable to increased
organic matter content in the revegetated soils, particularly in the 12-year revegetated soils,
due to increased decomposition rate of aboveground litter.
It is however suggested that future research activities should examine how the revegetation
strategy affects micronutrient dynamics in revegetated mined soils.
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